1. Field of the Invention
This invention relates to mounting devices for connecting a part of a wheel disc to a helical rail secured to a wheel rim. The invention is of particular application to variable track wheels of agricultural vehicles, such wheels being so devised that the vehicle track can be altered on site for adaptation to particular requirements, for example dictated by the cultivation of crops.
2. Description of the Prior Art
In one of the commonest systems, each wheel has a wheel disc fittable to a wheel shaft, a tyred rim and a number of mounting devices connecting regions distributed in spaced relationship around the disc periphery to helical rails secured to the rim inner wall. The mounting devices used for this kind of wheel usually take the form of a rigid stirrup secured to the disc periphery by means of a radially adjustable connection enabling the stirrup to be urged into engagement with the corresponding helical rail. Since the axial position of the rim relative to the disc is altered with the wheel already positioned on the vehicle, proper adjustment of the various connections was a difficult matter because of vehicle weight, and consecutive adjustments of the wheel caused it to go out of true, with possible difficulties for on-road travel. When the radial adjustment of the connection is accompanied by slight axial displacement of the stirrup relatively to the disc, the out-of-roundness causes wheel buckling in the sense that the rim axis ceases to be dead parallel to the disc axis. Such buckling leads, inter alia on road surfaces, to heavy tyre wear.
Other suggestions, as disclosed by United Kingdom Patent Specification No. 755,301, are for mounting devices of the kind each comprising two co-operating jaws, the first jaw being rigidly secured to the disc periphery at the region concerned while the second jaw is movable and connected to the first jaw by an adjustable screwed connection enabling the jaws to clamp the rail between them at the chosen region. Since in this case the adjustment of rim centring on the disc is the result of co-operation between mating inclined surfaces on the rail and the stationary jaw, the action of clamping the jaws is accompanied by radial and axial movement of the mounting device relatively to the rail; clearly, therefore, none of the disadvantages mentioned is obviated. Also, the helical rail for use with such a facility must be of trapezoidal cross-section, and it is a delicate matter to make up and/or secure such a rail to the rim inner wall. Since the present normal construction entails the use of a rectangular cross-section rail, such a device is fundamentally unsatisfactory since it would lead to bruising or hammering and rapid deterioration either of the rail to which it was clamped or of the fixed jaw, with the result of undesirable backlash.